The present invention relates generally to zinc-bromine batteries. More particularly, the present invention relates to improved zinc bromides and quaternary ammonium bromides for use in such batteries, methods of making same, and batteries utilizing same.
A zinc-bromine battery is a type of bipolar, electrochemical flow battery, which is capable of collecting and discharging electric charge. A zinc-bromine battery typically includes a series or stack of voltaic cells, a pump for pumping electrolyte through the cells, terminal electrodes electrically coupled to the stack of cells, and stud terminals electrically coupled to the terminal electrodes and through which electric charge flows into and out of the battery.
The operation of zinc-bromine batteries is described, e.g., is U.S. Pat. No. 5,591,538 and in U.S. Pat. No. 5,650,239. As described therein, the battery cells are made up of a series of alternating electrodes and separators. The electrodes are said to be “bipolar” because an anodic reaction takes place on one side of the electrode and a cathodic reaction takes place on its opposite side. Therefore, each cell can be considered as having an anodic half-cell and a cathodic half-cell. An ion permeable separator separates the anodic half-cell from the cathodic half-cell. Electrolyte is pumped through each half-cell, i.e., an anolyte through the anodic half-cell, and a catholyte through the anodic half-cell.
The electrolyte in zinc-bromine batteries is an aqueous solution of zinc bromide and quaternary ammonium salts, for example, methylethylpyrrolidinium bromide, with optional supporting salts, such as NH4Cl, and it is circulated through the individual cells from external reservoirs. It should be understood that the battery may be in several states including a discharged state and a charged state.
In the discharged state, the anolyte is substantially chemically identical to the catholyte. During the process of collecting a charge, the following chemical reactions take place:Zn2++2e−→Zn2Br−→Br2+2e−Zinc is plated on the anode, and bromine is produced at the cathode. The bromine is immediately complexed by the quaternary ammonium ions in the electrolyte to form a dense second phase which is subsequently removed from the battery stack with the flowing electrolyte. Further, and when the battery is charged, zinc in stored on one side of each electrode and the complex bromine is stored in the catholyte reservoir.
During the electrical discharge process, the following chemical reactions takes place:Br2+2e−→2Br−Zn→Zn2++2e−In this reaction, zinc is oxidized, and the released electrons pass through the electrode where they combine with molecular bromine to form bromide ions. Further, the positively charged zinc ions travel through the separator and remain in solution, and at the same time, bromide ions pass through the separator in the opposite direction and remain in solution.
Zinc-bromine batteries have several advantages over other types of batteries. In particular, one such advantage is the relatively high energy storage capacity of a zinc-bromine battery. Even though zinc-bromine batteries are in many ways superior to other types of batteries, current commercial technologies for zinc-bromine batteries are not completely satisfactory. For example, specifications for the entire electrolyte system are stringent and require low part per million (“ppm”) levels of most of the transition metals; this being a quality issue to maintain recycle performance in the battery assembly after many charge/discharge cycles. The quaternary ammonium salt component typically must be fluid, capable of containing elemental bromine, and stable to the various operating conditions within electrochemical flow battery.
Commonly utilized classes of quaternary ammonium salts are dialkylpyrrolidinium halides, dialkylmorpholinium halides, and dialkylpiperidinium halides, including, e.g., methylethylpyrrolidinium bromide “MEP” and methylethylmorpholinium bromide “MEM”. However, all of these salts, unless used in suitable combinations with other components, allow formation of crystallite structure of the salts and the zinc bromides, which would ultimately short out the flow batteries if costly, time-consuming maintenance procedures are not performed.
Therefore, there is a need for zinc bromides that meet the purity standards for zinc-bromine batteries, a need for commercially suitable quaternary ammonium salts, and a need for zinc-bromine batteries that utilize the zinc bromides and quaternary ammonium salts.